1. Field of the Invention
The present invention generally relates to methods and apparatus for ultrasonically sensing vibrations and particularly to a method and apparatus for non-intrusively measuring low frequency vibrations using ultrasonic techniques.
2. Description of the Prior Art
The importance of quantitatively determining the vibration frequencies and amplitudes of structural systems and components is well known, both in the laboratory as a research tool and in the field as a monitoring and diagnostic technique. Of all the vibration-measuring instruments in existence, the accelerometer is the most commonly used because of its ease in application, ruggedness and economy. Nevertheless, the accelerometer has its limitations. Its sensitivity decreases with frequency and this sensor is not very useful when the frequency of vibration is below a few cycles per second. In addition, to be effective, the accelerometer must be mounted directly on the vibrating surface. Thus, it cannot be used, for example, to quantitatively measure the vibration of a pivoted valve disc inside a check valve, without disassembling the check valve. Further, it cannot be used to measure the vibration of very slender structures such as reactor control rods, because of their lower natural frequencies of vibration.
Strain gauges are also used, but again they must be mounted directly onto the vibrating surface and are thus again an "intrusive" vibration sensor. Furthermore, the strain gauge will not work if the vibration does not produce any strain, such as is the case in trying to determine the flutter of a pivoted valve disc in a check valve.
Displacement probes include both contacting and non-contacting types. They are another commonly used vibration sensor. The displacement probe needs a sturdy mounting surface close to the vibrating surface and measures the relative motion between these two surfaces. As such it is usually an "intrusive" vibration sensor. It is more difficult to install properly compared with the accelerometer; however, it does work even at very low frequencies and when the vibration does not produce any strain.
The dynamic pressure transducer is a very sensitive instrument used to measure pressure fluctuations in fluids. They can also be used indirectly to measure the vibrations of structures immersed in fluids, because a vibrating structure induces pressure fluctuations in the surrounding fluid. However, to mount pressure transducers, tap holes must often be drilled through the pressure boundary, again making the dynamic pressure transducer an "intrusive" instrument. In addition, dynamics pressure transducers are extremely fragile and deducing structural vibration amplitudes from the measured pressure fluctuation often requires sophisticated analysis.
Low frequency vibrations with fairly large amplitudes can often be visually "measured" either with the naked eye or with the aid of high-speed photography. This technique, however, requires the vibrating component to be accessible and visible and would again be "intrusive".
In view of the foregoing description of the present state of the art, it will be seen that none of the prior art devices provided a non-intrusive device that could monitor low frequency vibrations of encased or non-accessible elements let alone provide quantitative measurements of such vibrations.